1. Field of the Invention
The present invention relates to an analog multiplexer circuit for use in a semiconductor IC device, for differentially amplifying selected ones of a plurality of input differential analog signals and outputting the signals thus amplified. More particularly, the invention relates to an analog multiplexer circuit which has an active load circuit functioning commonly for a plurality of bipolar differential amplifying input circuits.
2. Description of the Related Art
A conventional analog multiplexer circuit comprises, for example, bipolar differential amplifying input circuits, an active load circuit, a selector circuit, and an emitter-grounded circuit.
Each of the bipolar differential amplifying input circuits has a pair of differential input bipolar transistors and a constant current source for supplying an electric current to the emitter node of the bipolar transistors. The bases of the bipolar transistors are connected to receive respective input signals, here termed differential analog signals.
The active load circuit is connected to the collectors of the differential input bipolar transistor of each input circuit, thus functioning commonly for the bipolar differential amplifying input circuits. Hence, the active load circuit simplifies the analog multiplexer circuit and serves to reduce the number of components required in the analog multiplexer circuit.
The selector circuit selects one of the bipolar differential amplifying input circuits, in accordance with a selection signal externally input, and supplies an operating current to the emitter node of the differential input bipolar transistors of the input circuit thus selected. The selected input circuit and the active load circuit constitute a differential amplifier circuit.
The emitter-grounded circuit generates and outputs a differentially amplified signal from the output of the differential amplifier circuit constituted by the selected input circuit and the active load circuit. The analog multiplexer circuit is disadvantageous, however. While one of bipolar differential amplifying input circuits remains in selected state, the differential input bipolar transistors of any other input circuit that is not selected operate in reverse direction. That is, for one of the input bipolar transistors, the collector potential is higher than the emitter potential (i.e., the ground potential) by the base-emitter voltage V.sub.BE (usually 0.6 to 0.8V) of the bipolar transistor incorporated in the emitter-grounded circuit. Consequently, a current flows from the collector to the emitter, not from the emitter to the collector as it should in normal condition. In other words, the collector current of one of input bipolar transistors of any input circuit not selected is drained out. An offset voltage is applied to the differential input bipolar transistor of the selected input circuit. This inevitably reduces the accuracy of the output level of the differential amplifier circuit constituted by the selected input circuit and the active load circuit.
To avoid this consequence, it is proposed that a diode be connected between that transistor of the selector circuit which is on, and the emitter node of the differential input bipolar transistors of the input circuit not selected, and be arranged in the forward direction of the current flowing from the collector to emitter of each input bipolar transistor of the input circuit not selected. If a diode is so connected and arranged, it and the transistor of the selector circuit will connect the emitter node to the ground. As a result, the potential of the emitter node of the input circuit not selected is set to a forward drop voltage V.sub.F of the diode. The bipolar transistors of the input circuit not selected can therefore be turned off and are prevented from operating in the reverse direction.
Even if a diode is so connected, the bipolar transistors cannot be turned off in some cases, due to some difference between the base-emitter voltage V.sub.BE of each bipolar transistor and the forward drop voltage V.sub.F of the diode. There is the possibility that the bipolar differential amplifying input circuits operate at the same time.